Electro mechanical actuators (EMAs) are used to allow mechanical devices to achieve motion such as rotational motion and linear motion. Applications for EMAs that produce rotational motion include servoactuators, valve actuators, and flight controls. These types of EMAs are typically driven by electric motors and utilize power trains to deliver mechanical advantage. Typically a high gear ratio (e.g., 5:1 or higher) is required to deliver adequate performance at a reasonable cost for a particular application. The high gear ratio means that multiple rotations of the motor are required to achieve the desired range of motion. EMAs that produce linear motion include solenoids, linear motors, and voice coil motors. EMAs of this kind are often driven in a direct drive mode.
A typical rotational motion EMA used for flight control (e.g., a flight control EMA used to pivot the fins of a missile) consists of an internal rotor motor (an internal rotor is used to minimize system inertia) and a power train (to provide the desired gear ratio).